1. Field of the Invention
The present invention relates to a corneal-shape measuring apparatus for measuring the shape of a cornea of an eye to be examined.
2. Description of the Related Art
An apparatus for measuring a corneal shape is known in which a multiplicity of ring pattern targets (indexes) are projected onto a cornea to be examined, a target (index) image formed on the cornea is picked up, and the corneal shape is measured on the basis of the picked-up image.
Conventionally, the projection of ring pattern targets (indexes) onto the cornea is effected by illuminating from behind a Placido plate having a target (index) portion on which annular light-transmitting portions and light-shielding portions (masking portions) are alternately formed concentrically, and its illuminating methods include those shown in FIGS. 7A to 7C.
In a method shown in FIG. 7A, an annular light source 101 constituted by a fluorescent lamp, a cold cathode-ray tube, a stroboscope tube, a neon tube, or the like is disposed in the rear of a Placido plate 100, an illumination space is provided in such a manner as to envelop the light source 101 by a reflecting member 102 and the Placido plate 100, and the light emitted from the light source 101 is reflected by the reflecting member 102, thereby making it possible to illuminate the Placido plate 100 efficiently.
In a method shown in FIG. 7B, instead of the annular light source 101 in FIG. 7A, a multiplicity of light-emitting diodes (LEDs) 103 are arranged annularly around an outer rim portion of the Placido plate 100, and the Placido plate 100 is illuminated by the reflection of the reflecting member 102.
In a method shown in FIG. 7C, a multiplicity of LEDs 111 are arranged annularly immediately behind the light-transmitting portions of a Placido plate 110 so as to effect illumination.
However, these illuminating methods have the following problems. In the case of the method shown in FIG. 7A, to illuminate the entire region of the Placido plate 100 uniformly, it is necessary to provide a relatively large illuminating space between the Placido plate 100 and the reflecting member 102, so that the apparatus becomes large in size. In addition, the Placido plate 100 formed of a resin can expand and deform due to heat generated from the annular light source 101, possibly reducing measurement accuracy. Although it is possible to form the Placido plate 100 of glass so as to prevent the deformation due to the thermal expansion, the apparatus becomes very expensive.
In the case of the method shown in FIG. 7B, the generation of heat can be suppressed by using the LEDs 103, but it is still necessary to provide a sufficiently large illuminating space between the Placido plate 100 and the reflecting member 102 in order to illuminate the entire region of the Placido plate 100, making the apparatus large in size. In addition, if the LEDs 103 are provided around the outer rim portion of the Placido plate 100 as shown in the drawing, the illumination of ring pattern targets (indexes) on the outer side of the Placido plate 100 is shaded, so that a large apparatus is required to project the ring pattern targets (indexes) over a wide range, and the clearance between the apparatus and the eye to be examined becomes short. Further, since a difference in the quantity of light occurs in correspondence with the distance from each LED 103, it is difficult to illuminate the Placido plate 100 uniformly, and hence it becomes difficult to detect edges of the ring pattern targets (indexes) projected onto the cornea, leading to a decline in the measurement accuracy.
In the case of FIG. 7C, since the LEDs 111 must be arranged annularly for the respective light-transmitting portions of the Placido plate 110, the LEDs 111 are required in a large quantity, and these LEDs 111, combined with a power source therefor, make the apparatus very expensive.